Materials that absorb infrared rays over the entire infrared wavelength range can effectively generate heat unlike typical infrared absorbing materials for infrared absorption sensors. Therefore, such materials are expected to be used as materials that convert infrared energy into thermal energy in the environmental and energy fields.
To generate heat by infrared absorption, it is desirable that light be absorbed over the entire infrared wavelength range, for example, at all wavelengths of near-infrared rays, mid-infrared rays, and far-infrared rays. To use the generated heat as a heat source, it is desirable that an infrared absorbing material itself have heat resistance. Only infrared absorbing materials composed of an inorganic material can satisfy both the conditions described above. However, typical inorganic materials cannot absorb light at all wavelengths of infrared rays. A material that transmits infrared rays in a specific narrow wavelength range has been known as a material that can absorb infrared rays. For example, it has been described that, by using a manganese oxide-based nano-porous crystal, infrared rays having particular wavelengths are transmitted and those having other wavelengths are absorbed (e.g., refer to PTL 1). However, such a method that uses a manganese oxide-based nano-porous crystal results in high production cost, and therefore has no versatility as an industrial method. In addition, it is unclear whether the material causing such infrared absorption would function as a heat-generating material.
The titanium oxide reserves in nature are larger than the reserves of noble metal oxides, and titanium oxide is a cheap material that is widely used in industries from those concerning general-purpose materials such as white pigment, photocatalysts, and paint to those concerning special fields of application such as dye-sensitized solar cells and light-responsive materials. Titanium oxide itself slightly absorbs infrared rays at certain wavelengths in a short- and long-wavelength infrared range, but does not absorb infrared rays in most of the wavelength range. However, it is believed that efficient absorption of infrared rays may be achieved by causing a certain amount of distortion on the crystal lattice structure of titanium oxide.